Below represents a differential measurement system for a data-acquisition:
As far as I understood, in a differential measurement system neither the outputs of the signals nor the inputs to the differential amplifier is tied to ground. That's why many places I read that this makes it immune to ground noise.
But there is still a measurement ground (AI GND and G) for the system.
My question is about the differential/instrumentation amplifier part:
Does such an amplifier above measures the:
1-) voltage difference between the point A and B directly(without any reference) ?
(voltage difference between A and G) - (voltage difference between B and G) ?
I can already see both above mathematically equivalent. But does sometimes this measurement ground G is also tied to earth ground which might be bouncing(?).
2-) Is the practice of connecting AI GND and G to prevent exceeding common mode voltages? What happens if AI GND and G are not connected and both signal and measurement side ground is not connected to earth?
3-) Should the point G never be tied to the "earth ground" in any side? If it is, what would be the consequences? Or is doesn't matter?